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United States Patent |
6,096,610
|
Alavi
,   et al.
|
August 1, 2000
|
Transistor suitable for high voltage circuit
Abstract
A method and an apparatus for forming a transistor suitable for a high
voltage circuit. In one embodiment, the transistor is formed without
adding any steps to an existing state-of-the-art CMOS process. A well is
implanted into a portion of a substrate such that the well has a higher
doping concentration than the substrate. A first diffusion region is then
implanted into the substrate such that at least a portion of the first
diffusion is disposed within the well. In addition, a second diffusion is
implanted into the substrate separated from the well such that the second
diffusion region is disposed entirely outside the well. A channel region
is disposed between the first and second regions and gate is disposed over
the channel region to form the high voltage transistor. Since the second
diffusion region is disposed entirely outside the well in the lower doped
substrate, a higher junction breakdown voltage is realized. Furthermore,
with the transistor layout described herein having at least a portion of
the first diffusion region disposed within the well, adequate device
isolation is also realized.
Inventors:
|
Alavi; Mohsen (Beaverton, OR);
Ghani; Tahir (Beaverton, OR)
|
Assignee:
|
Intel Corporation (Santa Clara, CA)
|
Appl. No.:
|
780415 |
Filed:
|
January 8, 1997 |
Current U.S. Class: |
438/286; 438/510; 438/514 |
Intern'l Class: |
H01L 021/336 |
Field of Search: |
438/142,179,199,197,286,279,280,FOR 190,FOR 175,514,510
|
References Cited
U.S. Patent Documents
3600642 | Aug., 1971 | Allison et al. | 257/343.
|
3883372 | May., 1975 | Lin | 438/286.
|
4394674 | Jul., 1983 | Sakuma et al. | 257/336.
|
4543594 | Sep., 1985 | Mohsen et al. | 357/51.
|
4689504 | Aug., 1987 | Raghunathan et al. | 307/449.
|
4730129 | Mar., 1988 | Kunitoki et al. | 327/525.
|
5112766 | May., 1992 | Fujii et al. | 438/179.
|
5252848 | Oct., 1993 | Adler et al. | 257/335.
|
5278787 | Jan., 1994 | Iwasa | 257/335.
|
5296409 | Mar., 1994 | Merrill et al. | 438/189.
|
5306656 | Apr., 1994 | Williams et al. | 438/283.
|
5334880 | Aug., 1994 | Abadeer et al | 327/526.
|
5369045 | Nov., 1994 | Ng et al. | 438/286.
|
5371394 | Dec., 1994 | Ma et al. | 257/335.
|
5374843 | Dec., 1994 | Williams et al. | 257/335.
|
5399917 | Mar., 1995 | Allen et al. | 327/436.
|
5414364 | May., 1995 | McCollum | 327/525.
|
5420450 | May., 1995 | Yoneda et al. | 257/401.
|
5434531 | Jul., 1995 | Allen et al. | 327/530.
|
5465054 | Nov., 1995 | Erhart | 326/81.
|
5514608 | May., 1996 | Williams et al. | 438/286.
|
5514980 | May., 1996 | Pilling et al. | 327/525.
|
5536666 | Jul., 1996 | Miller et al. | 438/179.
|
5574303 | Nov., 1996 | Terasima et al. | 257/401.
|
5578509 | Nov., 1996 | Fujita | 438/286.
|
5583454 | Dec., 1996 | Hawkins et al. | 326/81.
|
5661048 | Aug., 1997 | Davies et al. | 438/217.
|
5776811 | Jul., 1998 | Wang et al. | 438/264.
|
Foreign Patent Documents |
354069387 | Jun., 1979 | JP.
| |
353066181A | Jun., 1998 | JP.
| |
Other References
Wolf, S.; Silicon Processing for the VLSI Era vol. 3: The Submicron MOSFET,
Sunset Beach, CA, pp.636 637, Jan. 1995.
Wolf, S., Tauber R.N.; Silicon Processing for the VLSI Era vol. 1: Process
Technology, Lattice Press, Sunset Beach, CA, p. 124, Jan. 1986.
"Metal-to Metal Antifuses with Very Thin Silicon Dioxide Films", Zhang et
al., pp. 310-312, IEEE, 1994.
CMOS Technology and Devices, Masakazu, S., pp. 74-75, AT&T, 1988.
VSLI Technology, 2nd Edition, Sze, S.M., pp. 466-513, AT&T, 1988.
|
Primary Examiner: Niebling; John F.
Assistant Examiner: Lattin; Christopher
Attorney, Agent or Firm: Blakely, Sokoloff, Taylor & Zafman LLP
Parent Case Text
This is a divisional of application Ser. No. 08/623,673, filed Mar. 29,
1996 now abandoned.
Claims
What is claimed is:
1. A method for forming a transistor suitable for a high voltage circuit,
the method comprising:
forming a first diffusion region and a second diffusion region in a
substrate of a complementary metal oxide semiconductor (CMOS) integrated
circuit, wherein one of the first and second diffusion regions is a source
region, the other one of the diffusion regions is a drain region and a
channel is defined between the first and second diffusion regions;
defining a well implant block region wherein one of the first and second
diffusion regions is disposed within the well implant block region and at
least a portion of the other one of the first and second diffusion regions
is disposed outside of the well implant block region;
implanting a well in the substrate outside of the well implant block
region;
blocking a well implant in the substrate inside the well implant block
region; and
forming a gate above the channel.
2. The method described in claim 1 further comprising:
forming an insulative layer above the channel between the substrate and the
gate.
3. The method described in claim 1 wherein the first diffusion region is
the drain region and the second diffusion region is the source region.
4. The method described in claim 1 wherein the first diffusion region is
the source region and the second diffusion region is the drain region.
5. The method described in claim 3 wherein the source region comprises a
first portion and a second portion such that the source region is arranged
on the substrate such that the gate is formed over the first portion of
the source region and not over the second portion of the source region.
6. The method described in claim 5 wherein the first portion of the source
region is disposed between the second portion of the source region and the
drain such that no conductive path exists between the source and drain in
response to a first potential applied to the gate.
7. The method described in claim 4 wherein the drain region comprises a
first portion and a second portion such that the drain region is arranged
on the substrate such that the gate is formed over the first portion of
the drain region and not over the second portion of the drain region.
8. The method described in claim 7 wherein the first portion of the drain
region is disposed between the second portion of the drain region and the
source such that no conductive path exists between the source and drain in
response to a first potential applied to the gate.
9. The method described in claim 3 wherein source region is disposed in the
substrate around the drain region and wherein the channel is disposed in
the substrate around the drain region between the source region and the
drain region.
10. The method described in claim 4 wherein drain region is disposed in the
substrate around the source region and wherein the channel is disposed in
the substrate around the source region between the drain region and the
source region.
11. A method for forming an n-channel transistor suitable for a high
voltage circuit, the method comprising:
providing a p-type epitaxially grown substrate of a complementary metal
oxide semiconductor (CMOS) integrated circuit;
blocking a p-well implant over a blocked region on the substrate;
forming a drain within the blocked region in the substrate;
forming a source separated from the drain in the substrate such that a
channel is disposed in the substrate between the source and the drain, the
source at least partially outside the blocked region; and
forming a gate above the channel.
12. The method described in claim 11 wherein forming the source includes
forming the source around the drain such that the channel is formed around
the drain between the source and the drain.
13. The method described in claim 11 wherein forming the source includes
forming the gate above a first portion of the source, wherein the first
portion of the source is disposed between the drain and a second portion
of the source.
14. The method described in claim 13 wherein a gate oxide is formed above
the channel between the substrate and gate and above the source and drain.
15. The method described in claim 13 wherein the drain has a first junction
breakdown voltage and the source has a second junction breakdown voltage,
wherein the first junction breakdown voltage is greater than the second
junction breakdown voltage.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to integrated circuits and more
specifically, the present invention relates to high voltage transistors
used in integrated circuits.
2. Related Application
This application is related to co-pending application Ser. No. 08/623,969,
filed Mar. 29, 1996, entitled "Method and Apparatus for Providing High
Voltage With A Low Voltage CMOS Integrated Circuit," and assigned to the
Assignee of the present application.
3. Description of the Related Art
In the integrated circuit industry there is a continuing effort to increase
circuit speed as well decrease integrated circuit device sizes. Modern
microprocessors contain an increasing number of devices in smaller areas
of integrated circuit substrate. To achieve this increased packing
density, integrated circuit devices are "scaled" in size and reduced in
dimension. A consequence of this scaling is that oxide thicknesses and
junction breakdown voltages are reduced proportionally. Although reducing
the oxide thicknesses may be necessary when reducing the overall sizes of
transistor devices, the associated thinner oxides are problematic for high
voltage transistors.
Since the voltage levels that can be tolerated by state-of-the-art
integrated circuits is constantly decreasing, the maximum input and output
voltages that are generated by such integrated circuits is decreasing
correspondingly. Consequently, ordinary modern integrated circuits are low
voltage tolerant and are therefore unable to interface with high voltage
devices without suffering gate oxide or junction breakdown.
One application which requires the application of high voltages from a low
voltage tolerant circuit is the programming of discretionary connections
such as antifuse devices or the like. Antifuse devices are used as
programming elements in electrically programmable read only memory (PROM).
When an antifuse device is initially fabricated, it provides a high
resistance connection. However, after the antifuse is programmed, or
fused, it provides a permanent low resistance electrical connection.
Unfused antifuse devices are generally programmed with the application of a
high programming voltage and current. After the application of the
programming voltage and current, the antifuse is permanently programmed
from a high resistance to a low resistance electrical connection.
One problem with programming antifuse devices with low voltage tolerant
CMOS integrated circuits is that the voltages required to fuse the device
often exceed the maximum junction breakdown voltage limits of ordinary
CMOS integrated circuit transistors. As described earlier, with the trend
of integrated circuit device sizes being scaled down, the maximum gate and
junction breakdown voltages of many modern integrated circuit processes
are continuing to decrease. Consequently, modern prior art CMOS circuits
are unable to tolerate the high programming voltages required to program
antifuse devices without expensive modifications to the manufacturing
process.
Some prior art techniques of implementing low voltage tolerant circuits
capable of providing a high voltage output are described in U.S. Pat. No.
5,399,917 entitled "High Voltage Tolerant Switch Constructed for a Low
Voltage CMOS Process" and U.S. Pat. No. 5,434,531 entitled "High Voltage
Tolerant Switch Constructed for a Low Voltage CMOS Process." Each of the
above-listed prior art patents are assigned to the Assignee of the present
invention.
FIG. 1 is an example of one prior art integrated circuit 10 which is
capable of providing a high voltage output at node 29. Prior art circuit
consists of ordinary low voltage CMOS transistors including p-channel
transistor pair 20 and 21 coupled in series between VPP and output node
29. Resistor 25 and n-channel transistor pair 26 and 28 are coupled in
series between output node 29 and ground.
In the prior art embodiment shown in FIG. 1, VPP is a high voltage which
may be greater than that which prior art circuit 10 is designed to
tolerate across any individual transistor gate to drain, gate to source,
or between drain to source. Assuming n-channel transistor 28 is switched
on, output node 29 is pulled down to ground through n-channel transistors
26 and 28. Thus, assuming VPP is 7 volts, 7 volts are distributed across
p-channel transistor pair 20 and 21. With the substrate of p-channel
transistor 20 coupled to VPP, the substrate of p-channel transistor 21
coupled between the source and drain of transistors 20 and 21, and the
gates of p-channel transistors 20 and 21 coupled to 3.5 volts, there are
no p-channel transistor gate to drain, gate to source or drain to source
connections that are subjected to the entire 7 volts of VPP.
However, assuming n-channel transistor 28 is switched off, output node 29
is pulled up to VPP. Accordingly, 7 volts are distributed across n-channel
transistors 26 and 28. Although there are no n-channel transistor gate to
drain, gate to source or drain to source connections subjected to 7 volts,
the drain to substrate junction of n-channel transistor is subjected to
the entire 7 volts.
Consequently, the breakdown voltage of the drain to substrate junction of a
transistor configured such as of n-channel transistor 26 determines an
upper limit to the maximum amount of voltage which may be supplied by low
voltage integrated circuits such as prior art circuit 10. As the trend
towards smaller devices with higher packing density continues in the
integrated circuit industry, the problems associated with the breakdown
voltage of the drain or source to substrate junctions in transistors
becomes increasingly significant.
Therefore, what is needed is a high voltage transistor which would not
limit the maximum amount of voltage which may be supplied by low voltage
integrated circuits. To keep costs as low as possible, such a high voltage
transistor would be implemented in an existing state-of-the-art CMOS
process without adding any costly process steps, such as masking,
implantation or doping. The high voltage transistor would be useful in a
number of high voltage applications such as antifuse programming, or any
other application that requires interfacing low voltage integrated
circuits with high voltage devices. Other applications in which such a
high voltage transistor might be useful include EPROM programming,
interfacing with high voltage buses, driving liquid crystal displays etc.
SUMMARY OF THE INVENTION
A method and an apparatus for forming a transistor suitable for a high
voltage circuit is disclosed. In one embodiment, a well implant is formed
in a portion of a substrate wherein the well implant has a doping
concentration higher than the doping concentration of the substrate. A
first diffusion region is implanted into the substrate such that at least
a portion of the first diffusion region is disposed within the well
implant. In addition, a second diffusion region is implanted into the
substrate outside the well implant such that the second diffusion region
is spaced apart from the first diffusion region and a channel is formed
between the first and second diffusion regions. Finally, a gate is
disposed above the channel to form the transistor. With the second
diffusion region formed in the substrate outside of the well, a high
junction breakdown voltage is realized. Additional features and benefits
of the present invention will become apparent from the detailed
description, figures and claims set forth below. Other features and
advantages of the present invention will become apparent from the
accompanying drawings and from the detailed description which follows
below.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example and not limitation
in the accompanying figures.
FIG. 1 is a schematic of a prior art integrated circuit which provides a
high voltage from an integrated circuit.
FIGS. 2A and 2B are schematics of two embodiments of integrated circuits
which provide a high voltage output in accordance with the teachings of
the present invention.
FIGS. 2C and 2D are schematics of two embodiments of antifuse programming
and sensing integrated circuits in accordance with the teachings of the
present invention.
FIGS. 3A and 3B are illustrations of cross-sections of prior art MOS and
CMOS devices.
FIG. 3C is an illustration of a cross-section of MOS integrated circuit
devices in a substrate without a well implant.
FIG. 4 is an illustration of a cross section of a high voltage MOS device
in accordance with the teachings of the present inventions.
FIG. 5A is a layout view of a MOS transistor suffering from inadequate
source and drain isolation.
FIG. 5B is a top layout view of one embodiment of a high voltage transistor
in accordance with the teachings of the present invention.
FIG. 6 is an illustration of another embodiment of a high voltage MOS
transistor in accordance with the teachings of the present invention.
FIG. 7 is a top layout view of another embodiment of a high voltage MOS
transistor in accordance with the teachings of the present invention.
DETAILED DESCRIPTION
A method and an apparatus for forming a transistor suitable for a high
voltage circuit is disclosed. In the following description, numerous
specific details are set forth such as voltages, currents, etc. in order
to provide a thorough understanding of the present invention. It will be
obvious, however, to one having ordinary skill in the art that the
specific details need not be employed to practice the present invention.
In other instances, well known materials or methods have not been
described in detail in order to avoid unnecessarily obscuring the present
invention.
As indicated above, there are a variety of uses for low voltage integrated
circuits which can tolerate high voltages. An application that requires
high voltages and currents is the fusing of discretionary connections such
as antifuse devices.
An antifuse is essentially a thin oxide disposed between two layers of
conductor. In one embodiment of the present invention, an NMOS transistor
is used to form the antifuse by electrically coupling the source, drain
and substrate to ground with the gate of the transistor coupled to the
circuit provided by the present invention. By applying an appropriate high
programming voltage and current for a sufficient amount of time to the
gate of the antifuse device, a pinhole is formed in the gate oxide of the
transistor which causes the eventual breakdown of the antifuse. Before the
breakdown of the gate oxide of the antifuse, the antifuse forms a high
resistance electrical connection. However, after the breakdown of the gate
oxide, the antifuse remains in a permanent on state forming a low
resistance electrical connection.
In one embodiment of the present invention, an antifuse has a gate oxide
thickness of 3.5 nanometers. The gate oxide has an oxide breakdown voltage
of 5.5 volts. The junction breakdown voltage of the transistors made by
the same process is 6.5 volts for regular NMOS, 7 volts for regular PMOS
and greater than 10 volts for high junction breakdown voltage NMOS.
With an oxide breakdown voltage of 5.5 volts, it is noted that an
application of a programming voltage of at least 7 volts is required in an
acceptable period of time (well below 1 second) to permanently program the
antifuse from a high resistance condition to a low resistance condition.
The application of at least 7 volts, however, is greater than the maximum
voltage tolerable by the junctions of the MOS transistors. Consequently,
the antifuse is impossible to program and therefore implement with
peripheral circuitry made up of ordinary prior art CMOS transistors
without additional process steps.
High Voltage Integrated Circuits
FIG. 2A is a schematic of one embodiment of the present invention which is
a simple integrated circuit 30 capable of providing high voltages to
program an antifuse device. It is noted that the circuits described in
FIGS. 2A through 2D are also described in in co-pending application Ser.
No. 08/623,969, filed Mar. 29, 1996, entitled "Method and Apparatus for
Providing High Voltage With A Low Voltage CMOS Integrated Circuit," and
assigned to the Assignee of the present application.
In the embodiment shown in FIG. 2A, an unfused antifuse circuit may be
programmed with the application of approximately 8 volts for a period of
time. Circuit 30 features a pull up circuit coupled between VPP and output
node 38 and a pull down circuit coupled between output node 38 and ground,
or VSS. The pull up circuit includes p-channel transistor 32 coupled
between VPP and output node 38. The pull down circuit includes series
coupled n-channel devices 34 and 36 connected between output node 38 and
ground, or VSS. The gate of n-channel transistor 34 is coupled to receive
V.sub.PROTECT 35.
In the embodiment shown in FIG. 2A, VPP is about 8 volts and V.sub.PROTECT
35 is approximately one-half the voltage of VPP, or 4 volts. In response
to control signals 31 and 33, the p-channel transistor 32 and n-channel
transistor 36 may be placed in either conductive or non-conductive modes.
With p-channel transistor 32 in a conductive mode and n-channel transistor
in a non-conductive mode, p-channel transistor 32 pulls the voltage at
output node 38 up to approximately 8 volts. Thus, an antifuse circuit
connected to output node 38 would be programmed from a high resistance
condition into a low resistance condition.
If control signals 31 and 33 are set to place both p-channel transistor 32
and n-channel transistor 36 in a conductive mode, n-channel transistors 34
and 36 are configured to pull the voltage at output node 38 down to
approximately 4 volts. In the embodiment, an output of 4 volts is
insufficient to program an antifuse. Thus, an unfused antifuse circuit
connected to output node 38 would remain a high resistance condition.
With p-channel transistor 32 coupled between VPP and output node 38, and
with the voltage at output node 38 swinging between only 4 and 8 volts,
the substrate junctions of p-channel transistor 32 are never exposed to
excessive voltages in this embodiment. It is noted that the voltage levels
selected for control signal 31 are carefully selected in this embodiment
to prevent the gate oxide breakdown in p-channel transistor 32 and
n-channel transistors 34 and 36.
As will be described in greater detail below, circuit 30 features a special
high voltage n-channel transistor 34 which has an increased drain to
substrate breakdown voltage. As a result, the drain to substrate junction
of n-channel transistor 34 is able to tolerate the full 8 volts at output
38 circuit during antifuse programming. It is noted that in this
embodiment, the drain to substrate junction of an ordinary NMOS transistor
such as n-channel transistor 36 would be unable to tolerate the full 8
volts at output node 38 during programming.
As will also be discussed in greater detail below, special high voltage
n-channel transistor 34 is implemented in the process of the present
invention without the addition of costly processing steps. As a result, in
one embodiment of the present invention, special high voltage n-channel
transistor 34 has a near zero threshold voltage, or V.sub.T equal to
approximately 0 volts. In some cases, V.sub.T may be slightly greater or
less than 0 volts. It is appreciated that circuit 30 is implemented to
accommodate the near zero, or slightly sub-zero threshold voltage
characteristics of special high voltage n-channel transistor 34. That is,
circuit 30 can tolerate a situation where special high voltage n-channel
transistor 34 is not turned off even with 0 volts applied from gate to
source.
Therefore, with the present invention, the maximum output voltages which
can be provided by integrated circuits are no longer limited by the drain
to substrate junction breakdown voltage of ordinary n-channel transistors.
It is appreciated that for an output voltage swing of approximately 0 to 8
volts, circuit 30 can be easily modified to the embodiment shown in
circuit 40 of FIG. 2B. Similar to circuit 30 of FIG. 2A, circuit 40 of
FIG. 2B features a pull down circuit, including special n-channel
transistor 44 and ordinary n-channel transistor 46, coupled between
circuit output node 48 and ground, or VSS. A pull up circuit is coupled
between high voltage VPP and output node 48. The pull up circuit includes
series coupled p-channel transistors 42 and 49. The n-channel transistors
44 and 46 of circuit 40 are configured to pull the voltage at output node
48 down to approximately 0 volts. The gates of p-channel transistor 49 and
special n-channel transistor 44 are coupled to V.sub.PROTECT 45. In one
embodiment, V.sub.PROTECT 45 is approximately one-half the voltage of VPP.
In response to control signals 41 and 43, the voltage at circuit output
node 48 may be set to either 0 or VPP. With the two p-channel transistors
42 and 49 coupled in series between VPP and output node 48, the entire 8
volts of VPP may be distributed safely between the two p-channel
transistors.
FIG. 2C is a schematic of another embodiment of the present invention, CMOS
integrated circuit 201. Circuit 201 is configured to program and sense
antifuse device 207 with programming circuit 203 and sensing circuit 205.
Antifuse 207 is programmed with circuit 201 by applying a high programming
voltage and current to antifuse device 207 for a predetermined burn time
at node 249.
In the embodiment shown in FIG. 2C, programming circuit 203 includes a
differential cascode voltage switch forming two current paths. One of the
current paths is formed with series coupled p-channel transistors 211 and
215 and n-channel transistors 219 and 221. The other current path in
programming circuit 203 includes series coupled p-channel transistors 213
and 217 and n-channel transistors 265 and 267. Antifuse device 207 is
coupled to programming circuit 203 between node 249 and VSS 253. P-channel
transistor 209 is coupled between high programming voltage VPP 255 and the
gate and drain of p-channel transistor 211 and gate of p-channel
transistor 213. The gate of p-channel transistor 209 is coupled to input
voltage VCI 239 while the gate of n-channel transistor 221 is coupled to
write enable signal WR 243 and the gate of n-channel transistor 267 is
coupled to write disable signal WRDIS 269.
As will become more apparent in the discussion below, p-channel transistors
215 and 217 and n-channel transistors 219 and 265 act as "protect"
transistors for programming circuit 203. The gates p-channel transistors
215 and 217 and n-channel transistors 219 and 265 are coupled to protect
voltage VCCP 241. During high voltage operations, VPP 255 is coupled to
receive a high programming voltage sufficient to fuse antifuse device 207.
In addition, VCCP 241 and VCI 239 are coupled to receive half the
programming voltage received by VPP 255 during high voltage operations.
Sensing circuit 205 features a current mirror circuit which also forms two
current paths. One of the current paths of sensing circuit 205 includes
series coupled p-channel transistor 225 and high voltage n-channel
transistor 233 with its drain coupled to node 249 and antifuse device 207.
The other current path of sensing circuit 205 includes series coupled
p-channel transistor 227 and n-channel transistor 235. P-channel
transistor 223 is coupled between input voltage VCCR 245 and the gates of
p-channel transistors 225 and 227 at node 263. Pass gate 229 is coupled
between node 263 and the drain of p-channel transistor 225. The sources of
p-channel transistors and the gate of n-channel transistor 235 are coupled
to input voltage VCCR 245 while the gate of n-channel transistor 233 is
coupled to protect voltage VCCP 241. Pass gate 229 and the gate of
p-channel transistor 223 are coupled to sense enable signal 237.
In one embodiment, programming and sensing circuit 201 includes four
different modes of operation. The modes include idle state, writing,
sensing and non-burn write. The input voltage levels associated with each
mode are listed below in Table 1.
TABLE 1
______________________________________
Mode SENSE WR WRDIS VPP VCCP VCI VCCR
______________________________________
Idle State
0 0 0 1.8 1.8 0 1.8
Writing 0 1.8 0 8 4 4 1.8
Sensing 1.8 0 0 1.8 1.8 0 1.8
Non-burn 0 0 1.8 8 4 4 1.8
Write
______________________________________
While in idle state, both VCI 239 and sense 237 receive 0 volts which
deactivates the differential cascode voltage switch of programming circuit
203 and the current mirror of sensing circuit 205.
When in writing mode, SENSE 237 is set to 0 volts which deactivates the
current mirror of sense circuit 205. VPP 255 is set to the programming
voltage of 8 volts and VCI 239 and VCCP 241 are set to half the
programming voltage, or 4 volts, which activates the differential cascode
voltage switch of programming circuit 203. It is important to note that at
no time in one embodiment does the voltage difference between VPP 255 and
VCCP 241 exceed a safe voltage, which in the embodiment is about 4 volts.
WR 243 receives a logic high voltage of 1.8 volts placing n-channel
transistor 221 in a conductive mode while WRDIS 269 receives a low logic
level placing n-channel transistor 267 in a non-conductive mode. As a
result, node 251 is pulled down to a relatively low voltage close to VSS
while node 249 is pulled up to a relatively high programming voltage.
Therefore, an unfused antifuse circuit 207 connected to node 249 will be
programmed from a high resistance condition to a low resistance condition
during a write mode operation.
In one embodiment, a voltage of approximately 8 volts is supplied to node
249 and antifuse device 207 during a write mode operation. It is noted
that the special NMOS transistor having an increased drain to substrate
breakdown voltage described in greater detail below is utilized for
n-channel transistor 265. That is, a special n-channel transistor similar
to the n-channel transistors 34 and 44 of FIGS. 2A and 2B is used for
n-channel transistor 265. N-channel transistor 265 is designed to
withstand extraordinarily high voltages applied to the drain to substrate
junction. Accordingly, the high voltages applied to node 249 do not
destroy n-channel transistor 265.
At the same time, with p-channel transistor 215 coupled between node 251
and p-channel transistor 211, the voltage drop between VPP 255 and node
251 is dropped across both p-channel transistors 211 and 215. With VCCP
241 set to half the programming voltage, the gate oxide junction breakdown
voltage of 215 is not exceeded. In addition, with VCCP 241, node 257 is
biased to a level no lower than VCCP+V.sub.TP since a voltage lower than
VCCP+V.sub.TP would result in p-channel transistor 215 being switched off.
Therefore, the relatively high voltage drop is safely distributed across
both p-channel transistors 211 and 215 such that the maximum junction and
gate oxide breakdown voltages are not exceeded.
During a non-burn write operation, SENSE 237 is set to 0 volts which
deactivates the current mirror of sense circuit 205. VPP 255 is set to the
programming voltage of 8 volts and VCI 239 and VCCP 241 are set to half
the programming voltage, or 4 volts, which activates the differential
cascode voltage switch of programming circuit 203. WR 243 receives a low
voltage which places n-channel transistor 221 in a non-conductive mode
while WRDIS 269 receives a high logic level which places n-channel
transistor 267 in a conductive mode. As a result, node 251 is pulled up to
the high programming voltage instead of node 249. Node 249 is pulled down
to a relatively low voltage. Therefore, an unfused antifuse device 207 is
not programmed during a non-burn write mode operation and remains in a
high resistance condition, even though a high programming voltage is
applied at VPP 255.
It is noted that the a special NMOS transistor having an increased drain to
substrate breakdown voltage similar to n-channel transistor 265 described
above is utilized for n-channel transistor 219 to protect the drain to
substrate junction from the high voltage applied at node 251 during a
non-burn write operation.
During the non-burn write mode operation, with p-channel transistor 217
coupled between node 249 and p-channel transistor 213, the voltage drop
between VPP 255 and node 249 is dropped across both p-channel transistors
213 and 217. With VCCP 241 set to half the programming voltage, the gate
oxide junction breakdown voltage of 217 is not exceeded. In addition,
similar to the discussion with respect to node 257 above, node 259 is
biased to a level no lower than VCCP+V.sub.TP. Therefore, the relatively
high voltage drop is safely distributed across both p-channel transistors
213 and 217 such that the maximum junction and gate oxide breakdown
voltages are not exceeded.
During a sense mode operation, VPP 255, VCCP 241 and VCCR 245 are set to an
ordinary high logic voltages. VCI 239 is set to 0 volts which deactivates
the differential cascode voltage switch of programming circuit 203 while
SENSE 237 is set to a high logic value which activates the current mirror
of sensing circuit 205. Accordingly, corresponding currents will flow
through each path of the current mirror circuit of sensing circuit 205.
The amount of current which will flow through antifuse device 207 will be
directly proportional to the resistance of antifuse device 207. So, the
question of whether antifuse device 207 is in a high resistance condition
or in a low resistance condition is easily ascertainable by the amount of
current flow in sensing circuit 205 during a sense mode operation. In
particular, the voltage generated at output VOUT 247 will indicate the
particular condition of antifuse device 207.
It is appreciated that since the resistance of antifuse device 207 may
range from low resistance to high resistance, the sizes p-channel
transistors 225 and 227 and n-channel transistor 235 are carefully
optimized. In one embodiment, p-channel transistor 225 is sized small
enough to provide reasonable sensing margin which makes it insensitive to
any fluctuation in the resistance of a fused antifuse device 207, but
large enough to provide sense current for reliable sensing. N-channel
transistor 235 is sized much smaller than p-channel transistor 227 to
realize a high trip point level that is approximately between the voltage
levels at node 263 for antifuse device 207 in low and high resistance
conditions.
It is noted that since sensing circuit 205 is directly connected to
antifuse device 207 at node 249, sensing circuit 205 is also exposed to
the high programming voltages applied to node 249 during write mode
operations. Accordingly, another special NMOS transistor similar to
transistors 219 and 265 discussed above is utilized for n-channel
transistor 233. During high voltage operations, i.e. when VPP 255 receives
the programming voltage, the gate of n-channel transistor 233 is coupled
to VCCP 241 to receive half the programming voltage. Since special
n-channel transistor 233 is designed to tolerate high voltages at the
drain to substrate junction, the gate oxide and drain to substrate
junctions of special n-channel transistor 233 as well as p-channel
transistor 225 and pass gate 229 are not destroyed by the high voltages
applied to node 249.
It is also appreciated that since n-channel transistor 233 has a near zero
threshold voltage, or V.sub.T, as discussed above, the read margin of
sensing circuit 205 is increased accordingly. That is, the read margin of
sensing circuit 205 is not reduced significantly by the V.sub.T of
n-channel transistor 233.
In FIG. 2D, another embodiment of the present invention is shown as
programming and sensing circuit 202. Programming and sensing circuit 202
is similar to programming and sensing circuit 201 of FIG. 2C in that it
includes programming circuit 204 and sensing circuit 205. Programming and
sensing circuit 202 is also used to program and sense antifuse devices.
The sensing circuits 204 of programming and sensing circuits 201 and 202
are the same. Programming circuit 204 of FIG. 2D differs from programming
circuit 203 of FIG. 2C only in that n-channel transistors 265 and 267 and
write disable signal 269 have been removed. All other device connections
are generally the same.
Programming and sensing circuit 202 also features four modes of operation
including idle state, write, sense and non-burn write. The associated
voltage levels corresponding with each operating mode are listed below in
Table 2.
TABLE 2
______________________________________
Mode SENSE WR VPP VCCP VCI VCCR
______________________________________
Idle State
0 0 1.8 1.8 0 1.8
Writing 0 1.8 8 4 4 1.8
Sensing 1.8 0 1.8 1.8 0 1.8
Non-burn Write 1.8 0 8 4 4 1.8
______________________________________
With respect to the idle state and sensing modes, the operation of
programming and sensing circuit 202 of FIG. 2D is the same as programming
and sensing circuit 201 of FIG. 2C.
During write mode operations, n-channel transistor 221 is placed in
conductive mode with write enable signal WR 243 applying a high logic
value to the gate of n-channel transistor 221. Accordingly, node 251 is
pulled down to a relatively low voltage level while the voltage at node
249 is pulled high to a relatively high programming voltage level
sufficient to program an unfused antifuse device 207 from a high
resistance condition to a low resistance condition.
During a non-burn write mode operation, write enable signal WR 243 is set
to a low logic value and sense enable signal SENSE 237 is set to a high
logic level. Accordingly, n-channel transistor 221 is placed in a
non-conductive mode which causes node 251 to be pulled up to a relatively
high voltage and ties node 249 down to a relatively low voltage of VCCR
245 minus the threshold voltage of p-channel transistor 225, or
VCCR-V.sub.TP. Since VCCR 245 is set to only 1.8 volts during a non-burn
write operation, the voltage at node 249 is well below the required
programming voltage for antifuse circuit 207. As a result, an unfused
antifuse circuit 207 is not programmed during a non-burn write operation
with programming and sensing circuit 202, even though a high programming
voltage is received by VPP 255.
It is appreciated that n-channel transistors 219 and 233 for FIG. 2D are
also the special NMOS transistors having high drain to substrate breakdown
voltage as described above. With the n-channel transistors 219 and 233,
programming and sensing circuit is able to program the high voltage
antifuse device 207 using an ordinary state of the CMOS process. It is
noted that if ordinary CMOS n-channel transistors were used in place of
special NMOS transistors 219, 233 and 265, the drain to substrate
junctions of the transistors would be destroyed with the application of
the high programming voltage of VPP 255 during high voltage operations
such as write and non-burn write mode operations.
Special High Voltage NMOS Transistor
As indicated earlier, ordinary transistors of advanced state-of-the-art
CMOS processes have junction breakdown voltages that are usually
comparable or less than that of the gate oxide breakdown voltage. These
breakdown voltages limit the maximum allowed voltage seen by these
devices. Therefore, high voltage circuitry such as the antifuse
programming and sensing circuits described above are impossible to
implement with peripheral circuitry made up of ordinary prior art MOS
transistors.
FIG. 3A shows a cross section of two adjacent NMOS transistors in a
standard prior art process 301. A P- substrate 303 starting material is
provided and a P- well 305 is implanted therein. Adjacent transistors 327
and 329 are implanted in P- well 305 with isolation regions 307 isolating
the transistors from other devices on the substrate. Diffusion regions
308, 309, 311, 319 and 321 are formed in P- well 305 to form transistors
327 and 329. A thin insulating layer, or gate oxide, 317 is formed over
the substrate and then gates 315 and 325 are formed above the thin
insulating layer 317. Conducting channels 313 and 323 exist between the
corresponding source and drain diffusion regions of each transistor 327
and 329.
In general, the doping concentration of P- well 305 is higher than the
doping concentration of P- substrate 303. With a relatively high doping
concentration in P- well 305 in combination with an isolation region, or
field oxide region 307, sufficient isolation is provided to transistors
327 and 329 to shield the transistors from other devices on the substrate.
Without the adequate isolation, undesirable conducting paths, or channels,
would exist between neighboring diffusion regions. For instance, without
isolation region 307, there would be an undesirable conducting path
between neighboring N+ diffusion regions 311 and 319 of FIG. 3A.
FIG. 3B shows a similar standard CMOS process 330 cross-section with trench
isolation 337 between NMOS transistor 359 and PMOS transistor 357. As
shown in FIG. 3B, an additional N- well is selectively implanted into the
substrate to provide an N- doped well for p-channel transistor 357. N+
diffusion substrate connection 338 and P+ diffusion regions 339 and 341
are implanted into N- well 335 to form p-channel transistor 357. Similar
to the NMOS transistors 327 and 329 described in FIG. 3A, isolation region
337 in combination with the adequately doped N- and P- well regions 335
and 333 provide isolation for p-channel transistor 357 and n-channel
transistor 359. In other words, there are no undesirable conducting paths
between neighboring diffusion regions such as P+ diffusion region 341 of
transistor 357 and N+ diffusion region 349 of transistor 359.
With respect to the need for the present invention to provide a special
transistor with a high junction breakdown voltage, breakdown voltages may
be increased by reducing the doping concentration of associated substrate
materials. In other words, if a diffusion region is implanted into a
substrate with a lowered concentration of doping, the corresponding
junction breakdown voltage is increased. A significant consequence,
however, of reducing the doping concentration of a substrate into which a
diffusion region is implanted is that adequate isolation of the implanted
diffusion regions is severely compromised.
For example, referring to process 360 of FIG. 3C, diffusion regions 364,
365, 367, 375 and 377 are implanted into a P- substrate 361 with a low
doping concentration. With the low doping concentration of P- substrate
361, an increased junction breakdown voltage is realized by n-channel
transistors 383 and 385. Without adequate doping of P- substrate 361,
however, an undesirable conducting path 387 under field oxide region 363
exists. Thus, transistors 383 and 385 are not isolated from one another.
The consequence of the undesirable conducting path existing between
transistors is explained by the fact that with P- substrate 361 having a
low doping concentration, the threshold voltage V.sub.T of conducting path
387 through field oxide region 363 is less than 0 volts. Therefore,
conducting paths exist between transistors independent of voltages applied
to the transistor gates 371 and 381.
In FIG. 4, one embodiment of the present invention is shown. Process 401
utilizes an innovative layout arrangement for a well implant into a
substrate for transistor fabrication. It is appreciated that the
innovative layout of the present invention is realized by using the same
well implantation steps used in existing state-of-the-art CMOS processing.
That is, no costly processing steps must be added to existing
state-of-the-art CMOS processing to realize the present invention. In
addition, it is noted that even though process 401 is shown using P
substrate technology, the circuit may be reversed accordingly for
applications suitable for N substrate technology without departing from
the broader spirit and scope of the present invention. In that case, the
special high voltage device would become a P device and all of the signs
of the implants would be reversed.
In one embodiment the present invention, a P- well implant 405 is implanted
into a P- substrate 403. So, p- well 405 has a higher doping concentration
than P- substrate 403. In one embodiment of the present invention, P-
substrate 403 is formed from epitaxially grown P- type start material. In
that embodiment, P- well implant 405 receives greater impurity levels by
one to two orders of magnitude.
As shown in FIG. 4, the P- well implant 405 is blocked from the P well
implant blocked region 407A. Afterwards, diffusion regions 411, 413, 415,
423 and 425 are implanted into P- well 405 and/or P- substrate material
403 as shown. Field oxide regions 409 are formed as well as gate oxide 421
over the substrate. Gates 419 and 429 are formed over gate oxide 421 above
channels 417 and 427.
As shown in FIG. 4, n-channel transistors 431 and 433 are isolated from
each other with field oxide region 409 in combination with the P- well
implant 405. Since P- well 405 has a sufficiently high doping
concentration, field oxide region 409 adequately isolates transistor 431
from transistor 433. In particular, the threshold voltage V.sub.T of the
region between neighboring N- diffusion regions 415 and 423 is much
greater than the power supply voltage. As a result, there is no unwanted
conducting path between transistors 431 and 433.
In one embodiment of the present invention, N+ diffusion region 423 is
configured to be the source of transistor 433 and N+ diffusion region 425
is configured to be the drain of transistor 433. With P- substrate 403
having a lower doping concentration, it is appreciated that the drain to
substrate junction of transistor 433 has a higher junction breakdown
voltage in comparison with the junction breakdown voltages between N+
diffusion regions 413, 415, and 423 and P- well 405. It is also noted that
the source to substrate junction breakdown voltage of transistor 433 is
the same as the junction breakdown voltages of transistor 431. With
reference to the special NMOS transistors discussed in connection with
FIGS. 2A through 2D above, it is appreciated that special NMOS transistors
used in FIGS. 2A through 2D only require high voltage for the drain to
substrate junctions. Accordingly, a transistor such as n-channel
transistor 433 is ideally suited to be used as the special NMOS
transistors in FIGS. 2A through 2D above.
Therefore, with n-channel transistor 433, high drain to substrate junction
breakdown voltage is realized. In addition, with the P- well implant
blocked region 407A as shown in FIG. 4, transistors 431 and 433 are
adequately isolated from one another with field oxide region.
It is noted that P- well implant blocked region 407A may extend to boundary
407B as shown in FIG. 4 such that source diffusion region 423 is implanted
entirely in P- well 405. Drain diffusion region 425 still enjoys the high
junction breakdown voltage which provides the adequate high voltage
tolerance for high voltage circuit applications.
It is also noted that since no processing steps are added to the process
described herein, no extra implants are added to control the threshold
voltage, or V.sub.T, of n-channel transistor 433. As a consequence, the
V.sub.T of n-channel transistor 433 in one embodiment is near zero, or
even slightly greater or less than zero in some cases. Although this near
zero threshold voltage characteristic is undesired in some situations, the
circuits described above utilizing special high voltage n-channel
transistor 433 have been implemented to tolerate this characteristic.
FIG. 5A shows one possible top layout configuration of a transistor pair
531A and 533A in which a P- well implant has been blocked from a region
507A over transistor 533A. In FIG. 5A, only P- substrate material 503A
exists within P- well implant block region 507A and P- well material 505A
exists outside P- well implant block region 507A. Transistor 531A is an
n-channel transistor featuring diffusion regions 513A and 515A with gate
519A disposed over a channel region existing between diffusion regions
513A and 515A. Transistor 531A is an ordinary n-channel transistor with
ordinary junction breakdown voltages.
Transistor 533A also includes diffusion regions 523A and 525A with gate
529A disposed over a channel region existing between diffusion regions
523A and 525A. As shown in FIG. 5A, drain region 525A is implanted
entirely into P- substrate material 503A while source diffusion region
523A is partially overlapped by P- well implant blocked region 507A. With
gate 529A disposed over the corresponding channel region, conductive path
535A exists between source 523A and drain 525A. When zero volts are
applied to gate 529A, conducting path 535A is switched off. In addition,
the junction breakdown voltage between drain 525A and substrate 503A is
higher than the junction breakdown voltage of diffusion regions 513A,
515A, and 523A and P- well 505A.
A fatal problem with the layout of transistor 533A as shown in FIG. 5A,
however, is that conductive paths exists between the source 523A and drain
525A at the edges of transistor 533A. It is noted that thin oxide material
exists over the diffusion regions of CMOS process 501 A while field oxide
regions exist in the non-diffusion regions. As a result, field oxide
regions exist at the edges of transistor 533A. It is further appreciated
that at the edges of transistors 533A, the field oxide region exists in
low-doped P- substrate material 503A. Consequently, the threshold voltage
V.sub.T of the field oxide regions in P- substrate material 503A at the
edges of the transistor is less than 0 volts. As a result, undesirable
conductive paths 537A and 539A exist between source 523A and drain 525A,
even though transistors 531A and 533A are adequately isolated from one
another with P- well 505A and the corresponding field oxide region
existing between transistors 531A and 533A. Even with gate 529A coupled to
ground, conductive paths 537A and 539A cannot be shut off through the
field oxide regions existing at the edges of transistors 533A in P-
substrate material 503A since V.sub.T is less than zero volts at the edges
of transistor 533A. Consequently, transistor 533 does not function
properly.
FIG. 5B shows a top down layout view of transistor pair 531B and 533B in
which the problem with the unwanted conductive paths 537A and 539A of FIG.
5A have been corrected. As shown in FIG. 5B, P- well 505B has been
partially blocked over P- well implant blocked region 507B such that low
doped P- substrate material 503B exists inside P- well implant block
region 507B while P- well 505B exists outside P- well implant blocked
region 507B. Source 523B and drain 525B are implanted into P- substrate
material 503B and gate 529B is disposed over the channel region existing
between source 523B and drain 525B. Note that additional "wings" 541B and
543B have been added to source 523B such that the source diffusion region
of transistor 533B extends into the P- well 505B region and that gate 529B
is overlapping portions 545B and 547B of wings 541B and 545B,
respectively, as shown in FIG. 5B.
As described earlier, thin oxide exists above all diffusion regions in CMOS
process 501B. So, thin oxide exists in overlapped regions 545B and 547B.
Therefore, the threshold voltage V.sub.T associated with the overlapped
regions 545B and 547B is greater than 0 volts. As a result, the conducting
paths 537B and 539B as well as conducting channel 535B may be switched off
with gate 529B by applying zero volts overlapped regions 545B and 547B
with gate 529B.
It is noted that the paths formed by conducting paths 537B and 539B may be
considered as parasitic transistors operating in parallel with the main
device controlling conducting path 535B. The three transistors share
common gate 529B. When conducting path 535B is switched on, conducting
paths 537B and 539B are also switched on. Moreover, the main conducting
path 535B and the parasitic conducting paths 537B and 539B may be switched
off with common gate 529B.
Therefore, with diffusion regions 541 B and 543B extending to an area in P-
well 505B outside the P- well implant blocked area 507B, proper source to
drain isolation is realized. The high voltage transistor 533B has an
increased drain to substrate junction breakdown voltage while adequate
isolation is achieved as shown in FIG. 5B.
It is appreciated that an alternate embodiment of the present invention may
be realized with diffusion 523B configured to be a drain and diffusion
525B configured to be a source. In this alternative embodiment, the source
would have a high source to substrate junction breakdown voltage and the
drain would have an ordinary drain to substrate junction breakdown
voltage.
In FIG. 6, another embodiment of the present high voltage transistor 601 is
shown. Similar to the high voltage transistor shown in FIG. 4, a P-
substrate start material 603 is provided and a P- well 605 is implanted
into P- substrate 603 with the P- well implant blocked from a P- well
implant blocked region 607. As stated above, blocking P- well 605 from
being implanted into P- well implant blocked region 607 adds no costly
process steps to the state-of-the-art CMOS process. In addition, diffusion
regions 611, 623A, 623B and 625 are implanted into P- well 605 and/or P-
substrate 603 as shown in FIG. 6. Field oxide regions 609 are formed in P-
well 605 and thin oxide 621 is formed over P- well 605 and P- substrate
603. Gates 629A and 629B are formed over channel regions 627A and 627B.
As shown in FIG. 6, transistor 633 is an edgeless transistor. So, there is
no isolation region, or field oxide 609, in P- well implant blocked region
607. As a result, there are no isolation problems since there are no field
oxide regions 609 formed in low doped P- substrate material 603 which have
a threshold voltage V.sub.T of less than zero.
In one embodiment, diffusion region 625 is configured as a drain and
diffusion regions 623A and 623B are configured as the source of transistor
633. In the embodiment, gates 629A and 629B are coupled together such that
when a high logic voltage is applied, conducting channels 627A and 627B
existing between drain 625 and source regions 623A and 623B are switched
on. Conversely, when zero volts are applied to gates 629A and 629B,
conducting channels 627A and 627B are shut off.
FIG. 7 shows a top layout view of the embodiment shown in FIG. 6. P-
substrate start material 703 is used and a P- well 705 is implanted
outside P- well implant blocked regions 707. Thus, a higher doped P- well
705 is formed outside P- well implant blocked region 707 and low doped P-
substrate material exists inside P- well implant blocked region 707. Drain
diffusion region 725 is formed within P- well implant blocked region 707.
Source diffusion region 723 is formed around drain diffusion region 725 on
the substrate such that a region of P- substrate 703 material exists
between drain diffusion region 725 and source diffusion region 723. Gate
729 is formed over the region between drain 725 and source 723.
Accordingly, a conductive path 735 exists between source 723 and drain 725
and transistor 701 is formed. Conductive path 735 is shut off with zero
volts applied to gate 729. If a voltage higher than the threshold voltage
of conductive path 735 is applied to gate 729, then transistor 701 is
switched on.
It is noted that an alternative embodiment of transistor 701 may be
realized by configuring diffusion region 725 to be a source region and
configuring diffusion region 723 to be a drain. The alternative embodiment
would have a high junction breakdown for the source side and an ordinary
junction breakdown voltage for the drain side.
It is appreciated that the devices described in FIGS. 4-7 are fabricated
with significantly longer channel lengths compared to the minimum length
allowed for ordinary transistors to avoid reach-through. Without a P- well
implant in the channel regions under the gate area, the depletion regions
of such devices become much larger than in ordinary devices. Consequently,
reach-through would occur if depletion regions from the source and drain
overlapped. As a result, channel lengths are increased to further separate
the source and drain regions to avoid reach-through problems. Further, in
the embodiment shown in FIG. 5B, the size of the poly gate end caps is
also increased to avoid reach-through problems.
In sum, a high voltage transistor with a high junction breakdown voltage is
realized without adding additional costly masking, implantation or doping
steps. The described transistor is suitable for any high voltage
application using an ordinary low voltage tolerant CMOS process, such as
the circuits described in FIGS. 2A through 2D.
In the foregoing detailed description, an apparatus and method for forming
a transistor suitable for a high voltage circuit is described. The
apparatus and method of the present invention has been described with
reference to specific exemplary embodiments thereof. It will, however, be
evident that various modifications and changes may be made thereto without
departing from the broader spirit and scope of the present invention. The
present specification and drawings are accordingly to be regarded as
illustrative rather than restrictive.
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